START_USER_ID

Hi,When starting a process instance using businessProcess.startProcessByKey(processDefinitionKey), how can I set the user that instantiated the process? Right now, it's setting the START_USER_ID field in the ACT_HI_PROCINST table value to blank. Activiti explorer shows only the process instances that are instantiated by the current user logged in. So, when the database field value is blank, explorer won't show the process instance for any user.